Unveiling the influence of end-capped acceptors modification on photovoltaic properties of non-fullerene fused ring compounds: a DFT/TD-DFT study

Herein, unique A–D–A configuration-based molecules (NBD1–NBD7) were designed from the reference compound (NBR) by utilizing the end-capped acceptor modification approach. Various electron-withdrawing units –F, –Cl, –CN, –NO2, –CF3, –HSO3, and –COOCH3, were incorporated into terminals of reference compound to designed NBD1–NBD7, respectively. A theoretical investigation employing the density functional theory (DFT) and time-dependent DFT (TD-DFT) was performed at B3LYP/6-311G(d,p) level. To reveal diverse opto-electronic and photovoltaic properties, the frontier molecular orbitals (FMOs), absorption maxima (λmax), density of states (DOS), exciton binding energy (Eb), open-circuit voltage (Voc) and transition density matrix (TDM) analyses were executed at the same functional. Moreover, the global reactivity parameters (GRPs) were calculated using the HOMO–LUMO energy gaps from the FMOs. Significant results were obtained for the designed molecules (NBD1–NBD7) as compared to NBR. They showed lesser energy band gaps (2.024–2.157 eV) as compared to the NBR reference (2.147 eV). The tailored molecules also demonstrated bathochromic shifts in the chloroform (671.087–717.164 nm) and gas phases (623.251–653.404 nm) as compared to NBR compound (674.189 and 626.178 nm, respectively). From the photovoltaic perspectives, they showed promising results (2.024–2.157 V). Furthermore, the existence of intramolecular charge transfer (ICT) in the designed compounds was depicted via their DOS and TDM graphical plots. Among all the investigated molecules, NBD4 was disclosed as the excellent candidate for solar cell applications owing to its favorable properties such as the least band gap (2.024 eV), red-shifted λmax in the chloroform (717.164 nm) and gas (653.404 nm) phases as well as the minimal Eb (0.126 eV). This is due to the presence of highly electronegative –NO2 unit at the terminal of electron withdrawing acceptor moiety, which leads to increased conjugation and enhanced the intramolecular charge transfer (ICT) rate. The obtained insights suggested that the designed molecules could be considered as promising materials for potential applications in the realm of OSCs.


Introduction
][4][5] Some OSCs used fullerene-based acceptors which exhibit considerable advantages with almost 10% power conversion efficiency (PCE). 6Despite the remarkable success attained in fullerene-based OSCs, they encountered various challenges, such as expensive purication, inadequate stability and limited absorption in the visible wavelength range. 7,8In order to overcome these challenges, signicant research efforts have been RSC Advances PAPER focused on non-fullerene acceptors, particularly non-fullerene small-molecule acceptors (NF-SMAs). 8They offer distinct benets over fullerene derivatives, such as cost-effectiveness, tunable energy levels and effective absorption of visible light. 9ecent reports have demonstrated the NF-SMAs as robust alternatives to the fullerene acceptors, achieving comparable power conversion efficiency (PCE). 102][13][14] NFAs-based OSCs possess unique properties over the fullerene-based acceptors. 1513][14]16 The heterojunction formed by donor and acceptor moieties serves as the operative basis for organic solar cells (OSCs). 17Hence, PCE of solar cells primarily relies on the characteristics of the acceptor and donor materials. 18As the donor materials have undergone signicant development, the current emphasis is on enhancing the efficiency of acceptor moieties to achieve highly efficient OSCs. 19In recent years, signicant development is observed in the fused ring NFAs (FR-NFAs), which feature acceptor-donor-acceptor (A-D-A) architecture integrating a ladder-form fused donor core. 20,21mploying this molecular design approach, OSCs harvesting over 18% PCE are developed. 5,22herefore, in the proposed study a non-fullerene small molecule acceptor i.e., NTIC is utilized having A-D-A conguration with a central hexacyclic naphthalene-(cyclopentadithiophene) donor core and terminal acceptor group i.e., 2-(2,3-dihydro-3-oxo-1-H-inden-1-ylidene) propanedinitrile (INCN). 23In A-D-A type non-fullerene molecules, employing the planar p-extended donor core, such as naphthalene (cyclopentadithiophene), has the potential to enhance photon absorption, leading towards improved short-circuit current density (J sc ), hence making it a promising choice for the proposed research. 24Donor core shows a planar structure which is benecial for the p-electron delocalization and intramolecular charge transport (ICT). 25This helps to avoid aggregation in the solid form, making it suitable for efficient OSC devices. 23wing to these facts, NTIC is taken as a parent compound in this research paper.The structural modeling of NTIC into the reference compound (NBR) is accomplished by replacing the sulphur (S) with selenium (Se) atom and 1-methyl-4-hexylbenzene with methyl group to streamline the structure and avoid the computational cost.Consequently, various derivatives, denoted as NBD1-NBD7, are formulated by introducing different acceptor groups at the terminals of the NBR as shown in the Scheme 1.The aim of this research is to examine the inuence of various acceptor moieties on the photovoltaic characteristics of the naphthalene-based compound.
The DFT-based calculations are carried out for NBR and NBD1-NBD7 to investigate their FMOs, UV-Vis spectra, DOS, TDM, E b , V oc and ll factor (FF). Literature survey has revealed that the modication of the terminal acceptor moieties used to construct different compounds that shows improved photovoltaic and charge transfer properties.The designed molecules are expected to exhibit remarkable photovoltaic features, including minimum band gap, higher light absorption coefficient and elevated charge mobility.

Computational procedure
The present theoretical study was conducted utilizing the Gaussian 09 program 26 and visualization of the outcomes were accomplished via the Gauss View 6.0.soware. 27In order to select a suitable functional for current study, benchmark study was performed between reported experimental and DFT l max values at various functionals.For this purpose, the reference compound (NBR) was rst optimized at four different DFT functionals such as B3LYP, 28 CAM-B3LYP, 28 MPW1PW91 (ref.29) and M06 (ref.30) combining with 6-311G(d,p) basis set.Aer the successful optimization of NBR, the UV-visible analysis was conducted at the afore-mentioned functionals.The simulated l max values at above-mentioned functionals: B3LYP (674.189nm), CAM-B3LYP (525.799nm), MPW1PW91 (637.971nm) and M06 (635.258nm) were compared with reported experimental (657 nm) 23 results to choose an appropriate DFT functional for further investigation.This comparison indicated that B3LYP/6-311G(d,p) level exhibited close harmony with the experimental ndings as shown in the Fig. S1 † therefore, this functional was selected as the most suitable level for further analyses.

Results and discussion
In the present study, a donor molecule (NTIC) consisting of A-D-A conguration was employed to develop a reference compound (NBR).The NBR undergoes modication by substituting the sulfur atom (S) with selenium atom (Se) in the fused cyclopentadithiophene ring (donor) of NTIC and replacing its larger bulky alkyl groups (-C 6 H 13 ) with smaller methyl unit (-CH 3 ) to diminish the steric hindrance and to alleviate computational costs (Fig. 1).The reference chromophores (NBR) consist of two parts: central core (hexacyclic naphthalene-(cyclopentadithiophene)) as donor connected with two terminal acceptor moieties (2-(5,6-ouro-3-oxo-2,3-dihydro-1-H-inden-1y-yildene)propanedinitrile).Various electron withdrawing units -F, -Cl, -CN, -NO 2 , -CF 3 , -HSO 3 , -COOCH 3 were incorporated into terminals of reference compound to explore the photovoltaic properties.End-capped acceptors play a pivotal role in the design of high-performance OSCs, particularly in donor-acceptor (D-A) conjugated systems.These electron-decient moieties, attached to the ends of conjugated donor backbones, extend the p-conjugation and facilitate better charge delocalization.This structural modication allows for precise tuning of the HOMO and LUMO energy levels, optimizing the energy gap and enhancing light absorption.In OSCs, end-capped acceptors enable broader absorption spectra, increasing the generation of photogenerated excitons.They also improve exciton dissociation at the donor-acceptor interface, leading to efficient charge separation and transport.Consequently, the incorporation of end-capped acceptors into the molecular architecture signicantly boosts the efficiency and durability of organic photovoltaic devices.By the structure tailoring of NBR, seven new derivatives (NBD1-NBD7) were designed with same conguration as that of the parent and reference compounds (A-D-A).The (A-D-A) conguration, representing acceptor-donor-acceptor, is crucial in photovoltaic devices, such as organic solar cells, for several reasons.It enhances light absorption by broadening the absorption spectrum through molecular tuning of acceptor and donor materials.This structure also creates a strong internal electric eld that efficiently separates photo generated electron-hole pairs, reducing recombination losses.Additionally, it provides clear pathways for charge carriers, improving charge mobility and minimizing energy losses.These factors collectively boost key solar cell parameters such as power conversion efficiency (PCE), open-circuit voltage (V oc ), short-circuit current density (J sc ), and ll factor (FF), making the A-D-A conguration essential for high-performance solar cells. 27ll the designed compounds contain hexacyclic naphthalene-(cyclopentadithiophene) core paired with seven distinct acceptor moieties; 2- (5,6-

Frontier molecular orbitals (FMOs)
Frontier molecular orbitals (FMOs) study is an efficient approach to characterize the photovoltaic properties of the studied molecules.The FMOs diagrams facilitate the understanding of electronic density and the charge distribution pattern of HOMO and LUMO in a molecule. 37The HOMO (valence band) serves as an electron donor, while the LUMO (conduction band) acts as an electron acceptor. 38,39Effective charge transmission within a molecule requires the transfer of electron density from HOMO to LUMO. 40FMOs diagrams of reference (NBR) and tailored compounds (NBD1-NBD7) at B3LYP/6-311G(d,p) level are presented in the Fig. 3.The energy difference between HOMO and LUMO represents the band gap (DE = E HOMO − E LUMO ). 41The energy gap (DE) is crucial in evaluating stability, strength, hardness, soness and chemical reactivity of a molecule. 42Moreover, it has a signicant impact on the working efficiency of an OSC.The efficiency of OSCs increases with a smaller band gap and conversely decreases with a larger band gap. 43The charge carrier mobility in designed molecules (NBD1-NBD7) is enhanced by introducing electron withdrawing acceptor moieties.The energy data of HOMO/LUMO orbitals obtained from the FMOs analysis of derivatives are given in the Table 1.Whereas, the HOMO+1/ LUMO−1 and HOMO+2/LUMO−2 values along with their visual representation are recorded in the Table S9 and Fig. S3.† For NBR, the HOMO/LUMO energy values are obtained as −5.839 eV and −3.692 eV, accordingly, resulting in a band gap of 2.147 eV, slightly larger than that observed in its derivatives except NBD1.This band gap showed harmony with reported experimental value (1.82 eV) 23 indicating the suitable selection of functional.Calculated HOMO energies of (NBD1-NBD7) are    44 Among all, NBD4 exhibits the least DE value as compared to the reference and other designed molecules.This might be owing to the presence of electron withdrawing nitro group (-NO 2 ) at the end of acceptor moiety (2-(5,6-dinitro-3-oxo-2,3-dihydro-1-H-inden-1-y-yildene)propanedinitrile).Furthermore, this decrease in DE can also be attributed to the phenomenon of negative inductive effect (−I) and prolonged conjugation within molecule.The prolonged conjugation in aromatic rings results in a signicant charge transference from donor to acceptor moieties.Additionally, the energies and HOMO/LUMO band gap of NBD4 that showed least band gap among all designed derivatives at B3LYP functional was also investigated at PBE1PBE/6-311 G (d,p) functional.The comparative study between PBE1PBE/6-311 G (d,p) and B3LYP functionals showed that the DE value of NBD4 determined at PBE1PBE functional is 2.292 eV, higher than the DE value of 2.024 eV obtained at B3LYP functional.
Moreover, DE value of NBD1 is reported as 2.157 and this might be due to the incorporation of uorine (-F) atoms at the terminal acceptors (2-(5,6-diouro-3-oxo-2,3-dihydro-1-H-inden-1-y-yildene)propanedinitrile), which showed positive inductive effect (+I) and causes steric hindrance.Accordingly, second narrow band gap is seen in the case of NBD3 and NBD6, probably attributed to the existence of efficient electron withdrawing substituents such as: -CN and -SO 3 H group at the terminal end of acceptor units (2-(5,6-dicyano-3-oxo-2,3-dihydro-1-H-inden-1-y-yildene)propanedinitrile) and (2-(5,6-disulfo-3-oxo-2,3-dihydro-1-H-inden-1-y-yildene)propanedinitrile) respectively.The -CN and -SO 3 H groups attracted more electrons towards itself, causes an extended conjugation and increased the charge carrier mobility.Similarly, NBD2, NBD5, and NBD7 also exhibits smaller band gap than NBR, but slightly larger than other derivatives, owing to the lesser resonance effect in the molecule.The decreasing order of DE value for the reference and all derivatives are: From above discussion, it is found that our designed molecules showing remarkable performance in comparison to the reference molecule. 45

Chemical reactivity parameters (CRPs)
The HOMO-LUMO band gap is utilized to compute the global reactivity parameters (GRPs) such as: ionization potential (IP), 46 chemical potential (m), 47 electron affinity (EA), electronegativity (X), 48 global electrophilicity index (u), 49 global hardness (h), 50 global soness (s) 51 and charge transfer index (DN max ). 52oopmans' theorem 53 is widely used for the calculation of these parameters by employing the eqn (S1)-(S8), † and the outcomes of these parameters are presented in the Table 2.
The IP and EA represents the electron donating and electron accepting properties of molecules, respectively. 54The higher values of IP (6.078-5.873eV) and EA (4.053-3.716eV) of designed derivatives (NBD1-NBD7) as compared to NBR (5.839, 3.692 eV, respectively), showed the greater charge transference between donor and acceptor moieties.The decreasing order for both IP and EA values are: There exists a close relationship between the GRPs and energy gaps. 55Higher kinetic stability of a molecule correlates with a larger energy band gap between HOMO/LUMO. 56hemical potential, global electrophilicity, soness and hardness play a pivotal role in inuencing the reactivity, stability and polarizability rate of molecules.Molecules with a narrow band gap may be regarded as so, chemically reactive, less stable and vice versa.All the designed molecules elucidate greater soness (s) and lower hardness (h) values

Density of states (DOS)
The examination of DOS provides further insight into the distribution of electron density.It describes distribution pattern around HOMO and LUMO which is affected by the nature of electron-withdrawing acceptor moieties.Each structure is divided into two segments for DOS interpretation: the acceptor and the donor. 57The results of DOS analysis are studied by PyMolyze soware.Here, the acceptor contributes:  S12.† The DOS graphs represent each part of the molecule in a distinct color i.e., relative intensity of donor is represented in green, while the relative intensity of acceptors is denoted in red color as shown in the Fig. 4. In DOS graphs, the positive values signify LUMO, while, the negative values depict HOMO along the x-axis.In the reference molecule (NBR), the HOMO exhibits electron density distributed across the entire molecule, with a slightly higher concentration on the donor groups.Conversely, the LUMO primarily features electron density concentrated on the acceptor moiety, although there is also a portion of electron density associated with the donating groups.In contrast, in the designed compounds (NBD1-NBD7), the HOMO exhibits higher electron density on the cyclopentadithiophene (donor group) and lower density on the acceptor portion.Conversely, the LUMO displays greater electron density on the acceptor group and lesser density on the donor part.These ndings demonstrate the end-capped acceptor moieties are satisfactorily procient in terms of withdrawing effect.

Absorption properties
UV-Vis analysis is used for determining the nature of transitions and charge-transfer characteristics of a molecule. 58The photovoltaic properties have correlation with the excitation energy (E), oscillation strength (f os ), dipole moment and absorption maxima (l max ). 59,60The value of l max describes the exact energy of photon required for the excitation of an electron from the HOMO towards LUMO, f os represents the possibility of transition, while the E refers to the energy necessary for a transition to occur. 61Therefore, broader absorption at a higher l max , high f os and low excitation energy are anticipated to yield efficient intramolecular charge transfer (ICT). 62The absorption spectra of reference molecule (NBR) and designed compounds (NBD1-NBD7) are calculated in both the gaseous and chloroform solvent phases.The representative values of l max along with their corresponding absorption parameters such as transition energy (E), oscillator strength (f os ) and major molecular orbitals contributions are shown in the Table 3. While, the remaining values are recorded in the supplementary part (Tables S10 and S11 †).The visual representation of the UV-Vis absorption spectra in both media is shown in the Fig. 5.
All the designed derivatives (NBD1-NBD7) exhibited higher maximum absorption values (l max ) as compared to the reference (NBR) which might be attributed to the mutual effect of auxochromes and chromophores incorporated in the NBD1-NBD7 compounds.Moreover, the absorption shi towards longer wavelengths is more prominent in case of the chloroform solvent which indicated that polarity induces the enhanced charge generation capacity.The values of l max of NBD1-NBD7 compounds in the solvent chloroform and gas phase are noted in the range of 671.087-717.164nm and 623.25-653.404nm, accordingly.Whereas, the l max for reference (NBR) compound in solvent and gas phases are 674.189and 626.178 nm, respectively.The absorption maxima (l max ) of NBD1-NBD7 in solvent phase are found in the following decreasing order: NBD4 (717.164)> NBD6 (710.099)> NBD3 (709.937)> NBD5 (689.562)> NBD7 (683.480)> NBD2 (681.827)> NBR (674.189)> NBD1 (671.087) in nm.Similarly, the following absorption maxima trend is seen in gas phase: NBD4 (653.404)> NBD6 (652.372)> NBD3 (652.132)> NBD5 (637.053)> NBD7 (632.149)> NBD2 (632.149)> NBR (626.178)> NBD1 (623.251) in nm.Moreover, the investigated molecules (NBR and NBD1-NBD7) exhibit lower corresponding excitation energies (E) as 1.839, 1.848, 1.818, 1.746, 1.729, 1.798, 1.746 and 1.814 eV, respectively, in the chloroform and 1.980, 1.989, 1.961, 1.901, 1.898, 1.946, 1.901 and 1.961 eV in the gas phase, respectively.Among all the designed molecules, NBD4 exhibits the highest l max in both media (717.164nm in the chloroform and 653.404 nm in the gas) with lower excitation energy values of 1.729 eV (in chloroform) and 1.898 eV (in gas phase).This might be due to the presence of -NO 2 group at the terminal acceptor moiety which has strong resonating electron withdrawing effect. 63Furthermore, its lower excitation energy leads to an increased charge transference from the HOMO to LUMO.Whereas, NBD1 exhibits the lowest l max of 671.087 nm (in solvent chloroform) and 623.251 nm (in gas phase), with the highest excitation energy (E) as 1.848 eV (in solvent) and  the absorption maximum l max for designed acceptor molecules (NDT1-NDT4) is 430.2 nm, 449.8 nm, 473.9 nm, and 444.9 nm, respectively.While the current research features broader and longer wavelength absorption ranging from (671.087-717.164nm) demonstrate excellent optical properties.Therefore, it is anticipated that all the designed compounds exhibit signicant optical properties, with high efficiency at low excitation energies in the absorption spectrum.

Transition density matrix (TDM) and exciton binding energy (E b )
The TDM is employed to estimate and analyze the electronic charge transfer in the excited state.5][66] It offers a comprehensive insights into electronic transitions taking place within a photovoltaic material. 67TDM plots for transitions in the rst excited state (S 1 ) are represented in the Fig. 6.The hydrogen atoms are mainly neglected by default, due to their minimal contribution to transitions.The investigated molecules are divided into two parts: acceptor (A) and donor (D) whose electron coherence regions are separated by the horizontal and vertical lines.The plots illustrated the accumulation of holes and electrons of the exciton in donor and acceptor part.The electron density in the reference molecule (NBR) is mostly present on the diagonal of the donor part.Whereas, in all the designed compounds (NBD1-NBD7), the distribution of electronic density is mainly concentrated on both the acceptor and donor moieties (mostly on the acceptor part) along diagonal and off-diagonal portions, facilitating the efficient charge transference from donor to acceptor moiety.This efficient charge transfer phenomenon is owed to the prolonged conjugation of utilized acceptor moieties.
Binding energy (E b ) signicantly inuences the photovoltaic properties, excited separation rate and efficiency of the  OSCs. 67The E b represents the energy needed to dissociate excitons into free charge carriers. 68It is performed to calculate the coulombic force of interaction between electrons and holes in a molecule.Also, it is directly proportional to the band gap (E H-L ) and coulombic interaction between electrons and holes and inversely proportional to the exciton dissociation rate. 69The lower the binding energy (E b ) and coulombic interactions, the higher will be the dissociation rate in excited where Open circuit voltage (V oc ) and fill factor (FF) Open circuit voltage (V oc ) is one of the important parameters to estimate the efficiency of OSCs. 47It refers to the maximum voltage produced by the photovoltaic devices to the external circuit when operating at zero current.In OSCs, electricity is produced at the donor of HOMO, transferring electrons to the acceptor of LUMO. 71Hence, the V oc value primarily depends on the energy levels of the LUMO and HOMO of acceptor and donor, respectively. 72The higher value of HOMO and the lower value of LUMO should be required to attain high device performance. 20Moreover, the V oc is in direct relation with HOMO-LUMO band gap between the designed molecules and the polymer.The higher the band gap between HOMO and LUMO, higher will be the V oc value.In organic photovoltaic (OPV) devices, the open circuit voltage (V oc ) is inuenced by the HOMO-LUMO band gap of the donor and acceptor materials.
As the band gap decreases, the energy difference between the donor's HOMO and the acceptor's LUMO increases, potentially raising the V oc .This reduction in band gap also allows absorption of a broader spectrum of sunlight, potentially increasing the short circuit current density (J sc ) by generating more charge carriers. 73urthermore, the HOMO/LUMO energy gap of the acceptor and donor units directly increases the PCE values.In this study, the PC 71 BM acceptor polymer is chosen for the calculation of V oc of the donor-type designed compounds owing to its conrmed effective charge transference from the donor to acceptors.The V oc of the reference compound (NBR) and designed derivatives (NBD1-NBD7) is computed by using the Scharber's Equation. 18According to this eqn (2), difference between the HOMO of donor (NBD1-NBD7) and the LUMO of acceptor (PC 71 BM) denotes the V oc 20 subtracting 0.3 (an empirical factor).The V oc values of all the titled molecules are presented in the Table 5, while their visual representation is displayed in the Fig. 7.
The results demonstrate that all the tailored compounds (NBD1-NBD7) exhibit greater V oc values as compared to NBR reference, owing to their higher HOMO energy values.Notably, NBD6 and NBD4 exhibit the highest V oc values of 2.348 and 2.347 V, respectively due to efficient terminal acceptor moieties with planar geometries which allow the ultimate charge transference from donor to acceptor and enhance the conjugation.Therefore, NBD4 emerges as the optimal choice for solar cell applications.Its electron-pulling acceptor group, enhanced by the highly electronegative -NO 2 attachment, results in excellent photophysical, electronic, and photovoltaic properties compared to all other derivatives.A decreasing order of V oc values for all the studied compounds in V is as follows: NBD6 (2.348) > NBD4 (2.347) > NBD3 (2.329) > NBD5 (2.257) > NBD7 (2.178) > NBD2 (2.176) > NBD1 (2.143) > NBR (2.109).
Fill factor (FF) signicantly inuences PCE of organic photovoltaic (PV) devices. 74It primarily relies on the opencircuit voltage value. 75Higher V oc values enhance the ll factor, signicantly contributing to the system's efficiency. 76It can be computed by using the following eqn (3). 77 ¼  where K B , T, and e represent the Boltzmann's constant (8.61733034 × 10 5 ), temperature (298 K), and the elementary charge (xed at 1), respectively.The computed values listed in Table S13 † reveal high FF indicating promising PCE.

Conclusion
In conclusion, a quantum chemical study is performed on the newly designed naphthalene-based molecules (NBD1-NBD7) to explore their optoelectronic, photo-physical and photovoltaic properties.It is noteworthy to discuss that the molecular engineering is performed by incorporating the efficient electronwithdrawing units in NBR which signicantly improves the photovoltaic characteristics of the designed derivatives (NBD1-NBD7).They showed reduced energy gap (2.024-2.157eV) and broadened optical absorption in chloroform (671.087-717.164nm) and gas phase (623.251-653.404nm).Further, all derivatives showed higher V oc values compared to NBR which are calculated via the HOMO donor and polymer LUMO PC 71 BM .
Exploring the photovoltaic properties more particularly, it is reported that NBD4 is potent among all the molecules as it exhibits the maximum absorbance at 717.164 nm (chloroform solvent) and 653.404 nm (gas phase) with lowest binding energy value of 0.126 eV.Moreover, the efficient charge transfer from the HOMO of naphthalene-based donor to the LUMO of the end-capped acceptor units is demonstrated via the results of FMOs, DOS and TDM analyses.The results indicate that the designed NFAs-based compounds (NBD1-NBD7) are potential candidates for the next-generation photovoltaic devices.
Fig.2displays their chemical structures, while the optimized geometries obtained via the DFT analysis are represented in the Fig.S2.† However, their Cartesian coordinates are provided in the Tables S1-S8 (ESI).†

Fig. 1
Fig. 1 Conversion of NTIC into NBR by (I) replacement of thiophene with selenophene and (II); replacement of hexyl group with methyl group.
1.989 eV (in gas phase).From literature, Asif et al. investigated the optical properties of naphtho-dithiophene based nonfullerene acceptor molecule.The results demonstrates that

Fig. 6
Fig. 6 Transition density matrix plots of NBR and NBD1-NBD7 at S 1 state.

Table 1
Energies of the frontier molecular orbitals of the studied compounds in eV

Table 3
Wavelength (l max ), excitation energy (E), oscillator strength (f os ) and major molecular orbital assessments of the titled compounds (D1-D7) in the chloroform solvent and gaseous phases a Chloroform solvent.b Gas phase.
, E H-L demonstrates the HOMO/LUMO energy gap and E opt is minimum energy needed for transition from the ground state (S 0 ) to excited state (S 1 ).The E b values for reference and all the designed chromophores are presented in the Table4.NBD4 exhibits the lowest E b (0.126 eV) as compared to NBR reference and all other designed compounds, whereas NBD2 displays the highest binding energy (0.169 eV).Therefore, NBD4 shows the maximum dissociation potential due to weaker coulombic interactions between electron and hole.

Table 4
Calculated binding energy (E b ) of titled compounds a a Units in eV.

Table 5
Open-circuit voltage (V oc ) of the entitled compounds a